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Portfolio of Ameloblasts Sanjay Madhavan /J. Pharm. Sci. & Res. Vol. 6(12), 2014, 392-395 Portfolio of Ameloblasts Sanjay Madhavan Student, Saveetha Dental College Chennai Abstract-Ameloblasts are cells present only during tooth development that deposit tooth enamel, which is the hard outermost layer of the tooth forming the surface of the crown. In the developing tooth the functioning ameloblast is a tall, narrow cell, with its base attached to the cells of the stratum intermedium. The nucleus is located basally and basal cytoplasm contains abundant mitochondria. The supranuclear cytoplasm contains a large, active Golgi complex and abundant rough endoplasmic reticulum, together with microtubules, which are predominantly longitudinally arranged, and secretory vacuoles which become larger and more numerous near the upper pole. At the upper pole the cell elongates into a single large Tomes’ process, and forms a fringe of smaller processes around its neck. The Tomes’ process contains numerous microtubules and large numbers of secretory vacuoles. The rough endoplasmic reticulum synthesizes various proteins and glycoproteins (including amelogenin and enamelin), which form the organic matrix of enamel (pre-enamel) and are packaged by the Golgi complex into secretory vacuoles. These then move into the Tomes’ process and the small neck processes, discharging their contents on to the surface. Mineralization of the matrix proteins by hydroxyapatite occurs almost instantaneously, producing small enamel crystallites and, with progressive mineralization, the enamel rods or prisms. Ameloblasts are derived from oral epithelium tissue The outer dental epithelium consisted of short flattened of ectodermal origin. Their differentiation from cells arranged as a multiple layer of cells on the labial preameloblasts is a result of signalling from surface of the apical loop. The cells contained a small, oval the ectomesenchymal cells of the dental papilla. The nucleus which was oriented perpendicular to the surface of ameloblasts will only become fully functional after the first the odontogenic organ. Outside the odontogenic organ were layer of dentin has been formed by odontoblasts. The cells cells and extracellular connective tissue elements. Near the are part of the reduced enamel epithelium after enamel outer dental epithelium several layers of fibroblast-like maturation and then are subsequently lost during tooth cells formed an investing sheath similar to the dental eruption[1,2] sac.The undifferentiated mesenchymal cells of the pulp facing the ameloblasts in this region were heterogeneous in ZONES OF AMELOGENESIS appearance. (1) Presecretory Zone (a) Region of Ameloblasts facing pulp,(ii) anterior portion The entire layer of ameloblasts from the point of greatest (odontogenic organ). convexity of the apical loop until a discrete layer of enamel There was an increased regularity of the junction between was present, was classified into various regions ameloblasts and provisional stratum intermedium. The cells prior to the zone of enamel secretion. of the provisional stratum intermedium were a mixture of (a) Region of Ameloblasts facing pulp,(i) posterior portion squamous and cuboidal shapes, but the latter, containing (odontogenic organ). large irregularly shaped nuclei, predominated. Outside the The apex of the ameloblast faced the the undifferentiated outer dental epithelium, small capillaries came into closer cells of the pulp and the base was adjacent to equally association with the odontogenic organ and disrupted the undifferentiated cells of the provisional stratum investing appearance previously shown by the cells of the intermedium. The stellate reticulum and outer dental dental sac. In the pulp, three changes were noted. epithelium were also undifferentiated. The ameloblasts at First, the cells adjacent to the ameloblasts became this stage were organized into an apparently stratified low progressively organized into cuboidal, then low columnar columnar epithelium with two or three staggered levels of and finally, in the region of ameloblasts facing dentin, into nuclei. The nuclei were large, oval and elongated in the a single layer of differentiated columnar odontoblasts. long axis of the cell. The junctions with the pulp at its apex Second, cells lying deeper in the pulp showed evidence of and the provisional stratum intermedium at its base were organization into two or three layers of flattened cells wavy and irregular. A basement membrane separated the forming a subodontoblastic layer Lastly, large, thin walled ameloblasts from the pulp. The term "provisional stratum capillaries came into close association with the developing intermedium" was applied to the layer of cells lying against subodontoblastic layer . the base of the ameloblasts. These cells were "provisional" because, first, they were continuous with the well- (b) Region of ameloblasts facing dentin organized layer of stratum intermedium seen further It was in this region that the ameloblasts developed all the incisally, and second, because it was often unclear as to morphological features of secretory ameloblasts except for whether a cell in this layer belonged to the ameloblasts, the Tomes, processes. At the apical limit of the region the stellate reticulum or to stratum intermedium itself. appearance of the ameloblasts was more regular than in the previous region, and by the end of the apical one-third of 392 Sanjay Madhavan /J. Pharm. Sci. & Res. Vol. 6(12), 2014, 392-395 this region, mitotic figures were no longer seen in ameloblasts. (3) Maturation Zone. Half-way along this region the cells began to elongate and This was a very long zone which occupied the remaining the nucleus as well as the mitochondria became located in incisal length of the enamel organ of the tooth. Along the the base. An apical cell web with obvious terminal bars length of this zone the amount of dentin increased became evident. At the incisal limit of the region the continually until the pulp became partially occluded. The ameloblasts were fully functional in secreting the initial odontoblasts continued to form a stratified layer and began layer of enamel. The oval nuclei of these cells remained to show signs of regression. The thickness of enamel perpendicular to the layer of mantle dentin and were seen to remained constant up to the point where it was lost by lie at two staggered levels.Mitotic figures in this group of decalcification. The enamel matrix at the conclusion of cells were seen one-third of the way into the region; after outer enamel secretion was homogeneous in staining. this they no longer showed division.. The thickness of the Gradually, rod profiles began to appear and then became stellate reticulum was reduced to one cell layer at the distinct in the region of nearly mature enamel [4] and incisal limit of the region. In the pulp, the previously finally faded rapidly in staining intensity and disappeared differentiated odontoblasts became angled with respect to completely as it became mature enamel. the ameloblast-dentinal junction such that the cell sloped (a) Region of postsecretory transition. toward the apex. Thin wall capillaries moved closer to the For a short distance incisally, immediately after outer odontoblasts as the subodontoblastic layer was lost. The enamel secretion, the ameloblasts remained tall columnar, predentin appeared as a coarse matrix containing bundles but changed their inclination and came to lie perpendicular of fibrils resembling von Korff's fibres and gradually to the surface of the enamel. The mitochondria remained in thickened in amount and became more homogeneous near the infranuclear zone. At this stage, the stratum the incisal limit of the region. intermedium gradually began to lose its organization as a distinct layer. Beyond this short region, the height of the (2) Secretory Zone ameloblast decreased rapidly until the cell was about half (a) Region of inner enamel secretion. its former height. Along with this shrinkage, the base of the The point of initial enamel secretion was marked by the ameloblast and the cells of the papillary layer contained presence of large spherical dark staining bodies between intracellular dense debris[5]. In addition, cells resembling the apices of adjacent ameloblasts. Gradually, the macrophages were occasionally seen in the papillary interdigitating portions of Tomes, processes were layer[6]. delineated by the prongs of enamel matrix and took on the (b) Region of maturation proper. characteristics of inner enamel secretion [3].The After the region of postsecretory transition, the ameloblasts arrangement is described as the "picket-fence" and consists were modified into cells of early maturation in which of processes cut at various angles as they course into the changes were evident at the base and apex of the cell. enamel. The nucleus of the ameloblast appeared to be Foamy vacuolization appeared at the apex of the cell and slightly inclined incisally relative to a perpendicular line the mitochondria began to migrate towards this region. The drawn to the dentino-enamel junction. In the stratum nucleus remained basally located, but the base of the cell intermedium the bilayered and irregular organization of the became irregular. This was caused by the exaggerated basal cells disappeared, and now contained a single continuous bulge and by the large intercellular spaces between the layer of cuboidal cells with large spherical nuclei. The bases of adjacent ameloblasts. The papillary layer increased stellate reticulum and outer dental
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